Packaging of electronic devices serves multiple functions including to protect the enclosed circuit die surface and to provide a stress release mechanism between the die and the printed circuit board. In addition, the package needs to be compatible with application requirements for small size, high density, and low cost. One well known and long used device packing arrangement is the lead frame package. Although this is a robust package, it is relatively too large for some newer applications such as many cell phone applications.
Circuit dies recently have been encapsulated at the wafer level before slicing to produce a significantly smaller package. A Wafer Level Chip Scale Package (WLCSP) extends the wafer fabrication process to include device interconnection and device protection processes to produce an electronic device package with the same footprint area and only slightly larger than the enclosed die. FIG. 1 shows an elevated bottom perspective view of a typical WLCSP 10 according to the prior art. A wafer die 11 contains the device circuitry 12. An array of solder ball electrical contacts 13 (also referred to as “bumps”) on the bottom surface of the WLCSP 10 connect the device circuitry 12 within the WLCSP 10 to an external circuit structure such as a circuit board. As compared to lead frame packages, WLCSPs are far smaller and less expensive to produce. However, WLCSPs may be somewhat less robust physically such that they are not suitable for all applications, and a significant place still remains for lead frame packages.
In one common application of electronic devices is as sensors for sensing environmental characteristics in the vicinity of the device. For example, an electronic device may include an infrared (IR) sensor which may typically be fabricated into a device die element to form an integrated circuit chip. The sensor typically is covered by a cap that protects the sensor while still allowing the sensor to sense the environmental characteristic. For example, the cap may have a transparent window that allows IR radiation to be sensed. U.S. Patent Application 20070063145, hereby incorporated by reference, described an IR sensor fabricated in a leadframe package, but provided no teaching or suggestion as to how an IR sensor might be fabricated in a WLCSP.